A Levels Chemistry

Most modern cars are fitted with three-way catalytic converters in the exhaust system.
Which three gases are removed by such a catalytic converter?
A carbon monoxide, hydrocarbons, nitrogen oxides
B carbon monoxide, carbon dioxide, nitrogen oxides
C carbon monoxide, nitrogen oxides, sulfur dioxide
D hydrocarbons, nitrogen oxides, sulfur dioxide


Choice A is correct because:

Three-way catalytic converters are specifically designed to target the three primary pollutants in car exhausts: carbon monoxide (CO), hydrocarbons (CH), and nitrogen oxides (NOx). 

Harmful gases/pollutants

Carbon monoxide is a toxic gas produced by incomplete combustion of fuel in the engine. 

Hydrocarbons, which are unburned fuel molecules, contribute to smog and can have detrimental health effects. 

Nitrogen oxides, formed from the high-temperature reaction between nitrogen and oxygen in the engine, contribute to acid rain and photochemical smog.

Conversion in a catalytic converter

A three-way catalytic converter uses two catalysts, typically made of platinum, palladium, and/or rhodium, to facilitate chemical reactions that convert these harmful pollutants into less harmful substances. 

The first catalyst promotes the oxidation of carbon monoxide to carbon dioxide (CO2) and hydrocarbons to water (H2O) and CO2. 

The second catalyst promotes the reduction of nitrogen oxides to nitrogen (N2) and oxygen (O2).

Choice B is incorrect because:

Carbon dioxide (CO2) is not a gas that is removed by three-way catalytic converters. Instead, CO2 is a product of the reactions that occur within the converter.

Choice C is incorrect because:

Sulfur dioxide (SO2) is not a gas that is removed by three-way catalytic converters. The primary focus of these converters is to reduce CO, CH, and NOx emissions.

Choice D is incorrect because:

While hydrocarbons and nitrogen oxides are removed by three-way catalytic converters, sulfur dioxide is not. The correct third gas is carbon monoxide.

Oxides of Nitrogen and Sulfur | 2009 - O/N - 9701/11 | Q18